1. Field of the Invention
The present invention relates to a burner apparatus and methods of burning fuel gas in the presence of air. More specifically, the present invention relates to apparatus and methods for air introduction and air damping in such a gas burner apparatus.
2. Description of the Related Art
The majority of process burners get the air required for combustion through “natural draft.” In such natural draft burners, air flows through the burner into a process heater because of the light negative pressure inside the heater firebox. As a result, the air side pressure drop over the burner is low, typically less than about 1 in H2O (about 249 Pa) and often from about 0.25 to about 0.6 in H2O (about 62 Pa to about 149 Pa). This low pressure means there is a lack of resistance necessary to correct flow maldistributions. Burners are sensitive to flow maldistributions, which create less than optimal flame conditions, and hence, can lead to higher emissions of carbon monoxide and NOX, uneven and longer flames, detrimental impact on flue gas recirculation patterns inside the firebox, and poor excess air control during turndown of burner duty.
Generally, such natural draft burners have one of three types of air inlets or registers. An early design uses two concentric metal cylinders. Each cylinder has slots. One cylinder is stationary while the other can be rotated. By rotating the cylinder, all or a portion of the slots on one cylinder can be aligned with the slots on the other cylinder, thus allowing more or less air to flow through the slots. A subsequent design uses only a single stationary cylinder with slots. In this design, each slot is fitted with a damper blade on a shaft. By adjusting the angle of the damper blade, more or less air is allowed through the slots. A third type of air register utilizes an air inlet box located on the side of the burner plenum. The air inlet box has a single or multi-blade damper. The angle of the damper blades determines the amount of air flow into the plenum.
These conventional registers have difficulty with flow maldistributions. The conventional cylinder designs are subject to unwanted air turbulence and spiral air flow. The air inlet box design relies on side entry that causes uneven distribution of air across the plenum and, hence, the burner. Additionally, damper blades as air flow regulators do not regulate air in a linear fashion and, thus, are difficult to adjust to achieve the desired air flow. Accordingly, because burners are sensitive to flow maldistributions, as discussed above, there are often problems with such burner systems such as higher than optimal carbon monoxide and NOx emissions. Further, it is difficult to achieve tight shutoff with damper blades and concentric cylinders even when they are fully closed. It is desirable to have a register that overcomes these difficulties.